This invention relates to electromagnetic locks, and in particular to a self-adjusting door-movement alert switch assembly which is operational while a door is magnetically locked.
Electromagnetic locks are commonplace in prior art security systems. These locks usually employ an electromagnet fixed to a door frame to attract and hold magnetically an armature plate fixed to a door hinged on the frame. Early electromagnetic lock designs permitted no movement of a door relative to its frame while the door was locked. An unauthorized attempt to open a door so locked went undetected until the lock was overpowered to break forcibly the armature plate away from the energized electromagnet. Detection after the lock was broken was unsatisfactory from a security aspect.
In most instances, however, a properly designed and installed electromagnetic lock could not be manually overpowered to open the lock. This was unacceptable in emergency situations, because it was imperative that immediate exit passage be possible through a door electromagnetically locked. This required opening the power circuitry to the electromagnet. However, the operating switch controlling power to the electromagnetic lock was visually located at a remote station manned by security personnel.
The foregoing operational deficiencies were overcome by altering electromagnetic lock designs so that the armature plate was no longer fixed to the door, but rather it was permitted to "float" movementwise a limited distance relative to the door. A door-movement alert switch was incorporated either on the door frame or on the electromagnet which responded to the limited movement of the door to complete electric circuitry which was capable of actuating an alarm, for example, at the remote security station. Security personnel could either take the necessary measures to thwart an unauthorized attempt to open the door, or alternatively power could be interrupted to the electromagnet to enable an immediate emergency exit.
The position and adjustment of the alert switch was fairly critical, because the switch had to effect contact transfer in response to a small movement of the locked door which was by necessity less than the limited movement of the door permitted by the floating armature plate. In many installations, the proper operational position of the alert switch could not be maintained due to settlement and warpage of either or both the door and the frame upon which it was hinged.